The asphalt used for paving refining and industrial uses is a solid or semi-solid bituminous material that is either naturally occurring, or derived from petroleum refining processes and includes paraffinic and aromatic hydrocarbons and heterocyclic compounds. Constituents of the binder, for example those seen in Table 1, vary based on the required characteristics and available binder material.
TABLE 1Typical Constituents of Asphalt Binders.Type of ConstituentExamplesExtenderssulfur, lignin, petroleum, vegetable oilsModifiersElastomersnatural latex, synthetic latex, block co-polymers, reclaimed rubber, natural rubber,styrene-butadiene, SBR, styrene-butadiene-styrene, SBS, crumb rubber/ground tirerubberPlastomerspolyethylene, polypropylene, ethyl-vinyl-acetate, EVA, polyvinyl chloride, PVCAnti-Oxidantsmanganese salts, lead compounds, carbonblack, calcium saltsAnti-Strip Agentsamines, limePetroleum Distillaterecycled and rejuvenating oils, kerosene,naphtha, low volatile-high flash point oils,mineral spirits
In paving, mineral aggregates such as crushed stone are typically mixed with asphalt materials, producing pavement-type products suitable for vehicular or related traffic, such as those seen in Table 2. In addition to asphalt use in road and highway applications, asphalt is a commonly used material for construction purposes, such as roofing materials, water and damp-proofing products, bridge decks, racetracks, airport runways, parking lots, bicycle paths, and port facilities. Asphalt alone, however, often does not possess all the physical characteristics desirable for many construction purposes.
TABLE 2Typical Constituents of Asphalt Concrete Used in Pavement Applications.Type of ConstituentExamplesAggregate Materialmineral aggregates, crushed concrete, flyash, sand, gravel, crushed stone, slags,screenings, recycled asphalt pavement,recycled asphalt shinglesBindersbitumen/asphaltAdditivescellulose fibers, synthetic mats and grids
The performance required of any asphalt material is determined by its end use and/or application and is gauged by one or more measurable properties. For example, asphalt materials used in the roofing context must be designed to perform several, somewhat diverse functions. In order to saturate and impregnate an organic fiberglass mat, polyester mats, or comparable base materials, roofing asphalt must be very fluid at processing temperatures. Once applied as part of a roofing material, including shingles, roofing, underlayment and various membranes, the asphalt should also retain its durability and/or weather resistance over a wide range of climatic conditions.
For instance, unmodified asphalt may exhibit a poor Performance Grade Rating (PG Rating) as a pavement binder for pavements designed for high vehicular traffic or loading or severe climate zones. Although the PG Rating for asphalt may widely vary, asphalt generally used in road pavement applications for standard vehicular use and moderate climate, exhibit PG Ratings of about 64-22, which indicates a 64° C. average, seven-day maximum and a −22° C., single day minimum pavement design temperature. When used as a road pavement material, asphalt is typically subjected to temperatures in excess of 64° C. measured twenty mm below the pavement surface and below −22° C. at the pavement surface. Temperatures and traffic conditions outside the pavement design range lead to enhanced deterioration of the asphalt pavement. Hence, it has, for some time, been an objective to broaden the PG Rating range of asphalt used in road-pavement applications; which is typically referred to as the useful temperature range.
The most common type of paving composition in the United States is hot mix asphalt (HMA), which is a pre-selected mixture of mineral aggregate and asphalt, commonly referred to as Mix Designs. There are numerous mix designs used to meet climatic conditions and vehicular traffic. The mineral aggregate particle size gradation and asphalt type providing the optimum set of performance properties is referred to as the Job Mix Formula, of which there are numerous variations to satisfy the requirements of the pavement to include: vehicular traffic, climate conditions, and useful life expectancy.
To broaden the PG range of the asphalt pavement, modifiers and additives are added to the asphalt. In addition to increasing the PG range of the asphalt, modifiers also can improve other qualities of the asphalt, such as its toughness, flexibility and durability characteristics, such as: resistance to oxidative aging (weathering), resistance to moisture, and improved adhesive properties. Typically, modifiers such as elastomeric and/or plastomer type polymer modifiers are added to molten asphalt and mixed for minutes to hours to produce a modified asphalt, commonly referred to as PMA, polymer-modified asphalt, or PMB, polymer-modified bitumen. Cross-linking agents may be concurrently added to the mix or added post-processing. For paving, the modified-asphalt is then routed to a mixer where aggregate is added to produce the hot mix asphalt (HMA). The hot mix asphalt is then taken to the construction site for use in paving equipment.
In roofing, the PMA/PMB is referred to as modified-bitumen compound, typically combined with mineral filler in batches and/or a continuous mixer. The filled modified-bitumen compound is then routed to the production line to make various products.
Some major problems associated with the performance of hot mix asphalts (HMA) pavements can be moisture susceptibility (striping), permanent deformation (rutting), bleeding, shoving, and cracking (thermal and fatigue). The asphalt binder is selected for the paving environment to provide sufficient stiffness to resist rutting at expected high service temperatures and enough flexibility to resist fatigue and thermal cracking at intermediate and low service temperatures. However, the wider the range of temperatures at which a binder must perform, the more difficult it is to span the range with an unmodified binder. There are a number of modifiers and/or additives that can be added to the HMA mixture that each address one or more of the aforementioned problems.
For example, liquid anti-strips and hydrated lime are additives that can be used to reduce moisture susceptibility problems. They alter the surface chemistry at the interface between the aggregate and the asphalt resulting in an improved bonding of the asphalt and aggregate. Polymers can be used to modify an asphalt to increase the high temperature stiffness of the HMA, which can reduce the probability of rutting, bleeding, and shoving. Polymers may be added to improve adhesion and cohesion properties. Polymers may be added to improve asphalt's service life. Polymers may be selected and used to impart elastomeric properties which can reduce thermal and fatigue cracking by allowing the modified asphalt to undergo repeated strains with recovery. Oils and extenders may be added to improve low or cold temperature properties.
Current paving methods in the United States use a hot asphalt mix that involves melting the asphalt in a large heated tank and adding 1 to 8 percent of an elastomer (Grzybowski, et al, U.S. Pat. No. 7,202,290). Typically, high dosages of elastomer are used globally, usually as a synthetic rubber such as styrene butadiene styrene block co-polymer (SBS). The SBS block co-polymer dissolves slowly in the asphalt due to viscosity differences when only stirring is used. The addition of the elastomers and other polymers is typically accomplished by first pre-mixing the modifier and heated asphalt in a heated tank, referred to as a wetting tank. The mixture is then pumped into and through a high shear mill, if needed, until the modifier/polymer is of suitable size for dissolution. The process is repeated where the modifier/polymer requires additional processing to form suitable sized material. The mixture is then pumped into an agitated curing tank to complete the solvation of the modifier. At this stage, a cross-linking agent may be employed to enhance the properties of the modified binder.
After the elastomer has dissolved, a cross-linking agent, such as sulfur in various forms, peroxide, or a transition metal can be added to cross-link the elastomer, which reduces the amount of elastomer and permits use of lower or higher molecular weight elastomers for the same end point. Adding the cross-linking agent, sulfur, at the same time as the SBS block co-polymer, results in immediate and localized cross-linking, which may manifest itself as agglomerated SBS clumps that do not dissolve easily and can require future blending/milling. If the cross-linking agent is added too soon to the mix, before the rubber has dissolved, an intractable mass of rubber is formed which is not dissolvable. If the cross-linking agent is added late, both time and money are then wasted. Further, extended exposure of asphalt to high temperatures affects the viscosity and characteristics of the asphalt binder. Hence, the addition of the cross-linking agent is critical to the correct make up and economics of the asphalt hot mix.
Asphalt extenders and/or additives, such as RREO and high flash oils, are used to lower asphalt's viscosity, improve selected performance properties, and improve low temperature properties. Other modifications, for example, inclusion of vegetable oil (Grzybowski, et al, PCT/US1997/004874) provide for improved low temperature performance of asphalt compositions. In particular, the vegetable oil, such as corn oil, peanut oil, sunflower oil, soybean oil, or a combination thereof, is present at a concentration of about 1.0 to about 6.0 weight percent as compared to the cement binder.
Study on Indiana highways showed styrene butadiene rubber (SBR), styrelf (PAC), and asphalt rubber (AR) performed well after 11 years (McDaniel & Shah, Asphalt Additives to Control Rutting and Cracking). Publication: FHWA/IN/JTRP-2002/29. Joint Transportation Research Program: Indiana Department of Transportation and Purdue University; West Lafayette, Ind. 2003). High temperature binder testing revealed that all of the selected modifiers stiffened the binder, which relates to pavement rutting.
The use of ground tire rubber (GTR)/crumb rubber (CR) in asphalt, i.e. bitumen, as a binder modifier when added to asphalt in particulate form without extensive processing results in an unstable mixture due to only partial dissolution, which upon storage at normal temperatures of 250° F.-400° F., separates with the GTR settling out of solution.
Additives and/or modifiers may improve singularly the high temperature properties or low temperature properties, provide elastomeric properties, adhesive properties, or anti-aging. However, no single modifier exists that addresses the multitude of asphalt application and aging issues or provides combined benefits of high and low temperature properties. As such, there is an unmet need to provide novel compositions of asphalt that possess enhanced aging and temperature tolerance properties.